(1) Field of the Invention
The present invention relates to a micro-dimensional measurement apparatus for optically measuring a dimension between opposed edges formed on a surface of a substrate, and more particularly, to such a micro-dimensional measurement apparatus wherein the measurement is carried out by optically scanning the substrate with a light beam having a predetermined light intensity distribution, such as a Gaussian distribution.
(2) Description of the Related Art
Recent developments in precision manufacturing demand a very high accuracy in the working of various precision components. For example, in the manufacturing of a precision component such as an integrated circuit, a magnetic head or the like, the working accuracy must be on the order of less than 1 .mu.m. Accordingly, there is a pressing need for a micro-dimensional measurement apparatus by which a fine precision component can be dimensionally measured with a high accuracy and reliability.
In this field, it is well known to dimensionally measure a fine object by an optical measuring system which includes a white light source for illuminating the fine object, a microscope for magnifying an image of the illuminated fine object, a television camera for reading optical information as video data from the magnified image, and a processor for processing the video data to calculate a dimension of the fine object.
For example, where the width of a fine gap formed between magnet portions of a magnetic head is measured by the optical measuring system, the area of the magnetic head which includes the fine gap to be measured is magnified by the microscope under illumination of the white light source, and then optical information is read by the television camera as a series of video data from the magnified image. The series of video data is processed by the processor so that an intensity pattern of the light reflected from the measured area is prepared with respect to a series of addresses of picture elements which are read by an image sensor of the television camera. In other words, the reflected light intensity pattern so obtained can be considered to be a function of a distance measured along a line which crosses over the fine gap at the measured area. Since the gap zone of the magnet head has a lower reflectivity than that of the magnet portions thereof, the reflected light intensity pattern has a minimum pea which corresponds to a middle point between the gap edges. In particular, the reflected light intensity pattern profiles a curve descending gradually toward a minimum peak and then ascending therefrom.
In this prior optical measurement system, to measure a width of the gap zone, a slice pitch which is obtained by slicing the reflected light intensity pattern at a predetermined slice level, is calculated by the processor. This slice pitch corresponds to the number of picture elements which are read by the television camera at the gap zone along the line crossing over the measured area. Accordingly, if a reference value is previously prepared, which is obtained from a known width of a sample gap in the same manner as mentioned above, it is possible to calculate a width of the gap zone from the measured slice pitch on the basis of that reference value. Nevertheless, the optical measurement system mentioned above suffers from drawbacks resulting from the use of the white light source. In particular, it is impossible to carry out the measurement with high accuracy and reliability because it is difficult to stabilize an intensity distribution of the white light source with the passage of time. Also, when a fine dimension on the order of less than 1 .mu.m is measured, the accuracy of the measurement is not at all satisfactory because it is very difficult to obtain a fine spot for the illumination from the white light source, so that the light picked up from the fine gap zone is affected by the light reflected from the remaining zone, except for the measured area.
British Patent No. 2147097 discloses another type of optical measurement system for dimensionally measuring a fine object, which system includes a laser light source for emitting a laser beam having a Gaussian distribution, an acoustic-optical device for stepwise deflection of the laser beam, to scan the fine object to be measured with the laser beam, a detector for detecting the laser beam reflected from the fine object, and a processor for processing the reflection data obtained from the detector to calculate a dimension of the fine object.
This prior optical measurement system is directed to measuring a dimension between opposed edges formed on a substrate by scanning the substrate with a laser beam under the condition that, when the laser beam is projected on an edge line of the opposed edges, a portion of the projected laser beam which is included at one side of the edge line is detected by the detector, but the other portion thereof which is included at the other side of the edge line is not detected by the detector. For example, if an element having a trapezoid cross-section is provided on the substrate, it is possible to measure a dimension between opposed edges of such an element by this prior optical measurement system because these edge fulfill the condition as mentioned above, in that when the laser beam is projected on the edge lines of the element, a portion of the projected laser beam impinging on an oblique face extending from each of the edge lines toward the substrate surface is not detected by the detector. Furthermore, this prior optical measurement apparatus is directed to a measurement of a dimension of the opposed edges, which dimension is greater than a spot diameter of the laser beam.
In this optical measurement system, when a substrate having the element as mentioned above is stepwise scanned with the laser beam under control of the acoustic-optical device, so that the laser beam crosses over each of the edge lines of the element, the light beams reflected from the substrate at the scanning steps are detected by the detector as a series of reflection data. The processor prepares a reflected light intensity pattern on the basis of the series of reflection data with respect to a series of deflection voltage values, each of which is applied to the acoustic-optical device to deflect the laser beam on each of the scanning steps.
Also, the processor arithmetically processes two portions of the reflected light intensity pattern which correspond to two zones including the edge lines, respectively, so that each of the two portions of the reflected light intensity pattern is represented by a three-dimensional function based upon the method of least squares. Each of the three-dimensional functions so obtained has a point of inflection which is in accord with the corresponding edge line, because a rate of change of the function (that is, the reflected light intensity) reaches a maximum when the peak of the Gaussian distribution of the laser bean is in accord with each of the edge lines during the scanning. It can be easily understood that a distance between the two points of inflection corresponds to a dimension to be measured between the opposed edges.
Accordingly, in this prior optical measurement apparatus, a dimension to be measured between the edges formed on the substrate is calculated by the processor on the basis of a distance between the two points of inflection, which are found by differentiating the three-dimensional functions to determine the maximum rates of change thereof.
Note, in general, it is possible to throttle a laser beam spot to the order of 1 .mu.m. As mentioned above, this prior optical measurement apparatus is directed to the measurement of a dimension of a fine object which is larger than a spot diameter of the laser beam. Therefore, the measurement of such a fine object can be satisfactorily carried out with a high accuracy and reliability. However, when a dimension to be measured between opposed edges of the fine object is less than 1 .mu.m, the accuracy and reliability of the measurement is considerably downgraded because a reflected light intensity pattern resulting from the fine object has two kink points at the two portions thereof which should be represented by a three-dimensional function based upon the method of least squares, as mentioned above. Since a rate of change of an intensity of the light reflected from the fine object is discontinuously varied at the kink points, it is impossible t properly represent the portions of the reflected light intensity pattern by the three-dimensional function. In particular, when the laser beam crosses over a dimension between the edge lines which is smaller than a spot diameter of the laser beam, the spot of the laser beam first passes through one of the edges lines, and the leading edge of the spot then abuts against the other edge line without completely passing through one of the edge lines. Thereafter, the spot of the laser beam bridges the edge lines, and then the tailing edge of the spot leaves one of the edge lines. Therefore, during the scanning, the rate of change of the intensity is discontinuously varied when the leading edge of the spot abuts against the other edge line and when the tailing edge leaves one of the edges lines.